Fragile X Syndrome is the most common form of familial mental retardation and affects about one in 2,500 children. The syndrome is characterized by the presence of a cytogenetically detectable fragile site in band q27.3 near the end of the long arm of the X chromosome which, if not the cause of the disorder, is closely associated with it. The diagnostic molecular genetics of the Fragile X Syndrome has been reviewed by Sutherland, G. R. et al. (Clinical Genet. (1990) 37:2-11). An additional review is found by Nussbaum, R. L. et al (Ann. Rev. Genet. (1986) 20:109-145).
Identification of the DNA spanning and including the fragile site has been reported by Kremer et al (Am. J. Human Genetics (1991) 49:656-661) and Heitz et al. (Science (1991) 251:1236). Characterization of the fragile site has indicated a particular region of instability within a 5.0 KB EcoRI restriction fragment, with the instability segregating with the Fragile X genotype (Yu et al., Science (1991) 252:1179). The region of instability has further been localized to a 1 KB Pst I fragment containing a p(CCG).sub.n repeat. The Fragile X genotype is characterized by an increased amount of unstable DNA that maps to the repeat (Kremer et al., Science (1991) 252:1711). The availability of the cloned DNA makes possible the use of the DNA as a probe to detect length polymorphism of the P(CCG).sub.n to characterize the genotype of an individual at that locus (Kremer et al., supra), thereby obviating problems with cytogenetic visualization at the fragile site (Webb et al., Prenatal Diagnosis (1989) 9:771-781).
Additional diagnostic tools are available in the form of polymorphic microsatellite markers linked to the fragile site at Xq27.3 (FRAXA). Richards, et al., (Am. J. Hum, Genet. (1991) 48:1051-1057) have described polymorphisms associated with length variation in dinucleotide microsatellite repeats in the vicinity of Xq27.3. These markers have a recombination frequency of 1% and 7%, respectively, in two-point linkage analysis in 31 Fragile X families.
Thus, the availability of cloned DNA spanning the fragile site provides reagents uniquely suited for the detection of the Fragile X allele in appropriate subjects. Furthermore, techniques of gene therapy could be used to replace or compensate for the pathologic Fragile X sequence in affected cell types.